Methods for protecting furnace parts and the like



United States Patent 3,196,056 METHODS FOR PROTECTING FURNACE PARTS AND THE LIKE Martin N. Ornitz, Wilkinsburg, Pa., assignor to Blaw- Knox Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Sept. 28, 1959, Ser. No. 842,620 4 Claims. (Cl. 148-126) This invention relates to methods for protecting furnace parts and the like and particularly to a method of protecting furnace parts and othe metal parts in which relatively volatile low melting metals or compounds of such metals are introduced or come into contact with such parts.

The problem of protectingfurnace parts against corrosion by volatilized low melting metals is faced by many segments of the metallurgical industry. One particular example hereafter specifically described is in the field of powder metallurgy. It is the practice in powder metallurgy to form articles from powdered metal in molds or dies which are lubricated and bonded by zinc stearate or similar metallic base lubricants and bonding agents. The articles produced by this practice are then treated in furnaces, the temperatures being in the neighborhood of 1475 to 2000 F. for ferrous parts. It is the general practice to place the articles in graphite trays, Inconel trays or similar trays resistant to high temperatures and to pass the trays through the treating furnace. However, this practice varies, and trays may or may not be used and the work may be passed on roller hearths, mesh conveyors or in various other ways. In passing the articles through the furnace, the zinc stearate volatilizes and the zinc utlimately deposits on the nearest cool surface. For example, the bearing shaft ends, the doors, door frames, wall supports and the like, are quickly coated with condensed zinc which rapidly corrodes the surface upon which it collects resulting in rapid, ultimate destruction. The condensed zinc is particularly damaging to roll shaft ends and the like so that these parts have a very short life and must be frequently replaced. This is obviously a serious problem and one which has remained unsolved until the present invention. I have discovered that this problem can be readily overcome and the life of rolls, shaft ends, door frames and the like greatly extended by the practice of my invention. I have discovered that by applying a coating of refractory metal oxide over the surfaces to be protected that this problem can be substantially solved. In a preferred practice, I find that by applying a coating of stabilized zirconia to the surface, I can substantially completely eliminate the corrosive effect of zinc in the furnaces Where zinc corrosion had previously been a very real problem. By stabilized zirconia, I mean zirconia which has been stabilized by the addition of calcium oxide or the like stabilizing agents. I have found that other refractory oxides, such as alumina and titania may similarly be used in substitution for stabilized zirconia in the practice of my invention.

The invention can perhaps best be understood by the following examples:

Example I In a furnace operating at approximately 1800 F. in the production of powdered metal articles formed with zinc stearate, I found that iron nickel chrome alloy shafts had a useful life of about two months. At the end of that time the shaft ends which were adjacent the bearing housings and therefore closer to the cold exterior of the furnace were corroded with zinc to the extent that they were no longer useful. I prepared a new set of shafts for this furnace of the same iron nickel chromium alloy but coated the shaft ends with a coating of stabilized zir- 31,196,955 Patented July 20, 1965 conia. This was accomplished by spraying the stabilized zirconia through a mctallizing gun onto the surfaces to be coated in the presence of a high temperature flame causing the zirconia to fuse into a thin film or coating over the parts to be protected. After eleven months of use in the same furnace environment the coated shafts show no signs of attack and are still in use. This means that the coated shafts have ta least 5 /2 times the useful life of the uncoated shafts.

Example II In the same furnace as Example I, I placed shafts coated with alumina as replacements for uncoated shafts. The alumina coated shafts were formed by coating the shafts with a thin coat of metallic aluminum and converting the aluminum in situ into alumina by heating in an oxidizing atmosphere. The alumina coated shafts show no signs of attack after six months use and are still in use. This means that they have at least 3 times the expected useful life of the uncoated parts.

Parts treated with other refractory metal oxides such as titania produce similar results.

I have found that the coating can be applied not only by spraying the refractory metal oxide onto the surface and fusing as in Example I but may also be applied by dipping the part to be coated in a bath of the coating material and fusing, by dipping into the molten pure metal and oxidizing in situ as in Example II and by spraying with metal and oxidizing or by any other known method.

In addition to treating parts for furnaces, I have found the technique of this invention may be used in processes where molten zinc is used, for example, hold down parts and olls for galvanizing pots.

Similarly the technique of this invention can be used for the protection of furnace parts for the purification of aluminum where the splash of molten hot aluminum striking the metal parts corrodes the parts.

I prefer to use stabilized zirconia as the coating material. However, it must be borne in mind that other refractory metal oxides could be substituted.

In the foregoing disclosure I have dealt with zinc and aluminum as the corrosive volatile metals. However, similar protection is offered against corrosion by other low volatile metals such as magnesium, sodium, lead and tin.

I prefer to apply a Nichrome undercoat in the form of a thin layer on the metal parts before applying the refractory metal oxide coating. This is particularly true where the coefiicient of expansion of the metal part is markedly different from that of the coating and the temperatures encountered are high. This is a matter in which men skilled in the art may use their own judgment.

I have set out certain preferred practices of my invention and certain preferred embodiments in the foregoing disclosure. However, it will be understood that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. In the process of treating materials at elevated temperatures in the presence of metal parts, which materials contain metals volatile at said elevated temperature and corrosive to the metal parts, the improvement which comprises coating said metal parts with a refractory metal oxide.

2. In the process of treating materials at elevated temperatures in the presence of metal parts, which materials contain metals volatile at said elevated temperature and corrosive to the metal parts, the improvement which comprises coating said metal parts with a refractory metal oxide selected from the group consisting of stabilized zirconia, aluminum and titania.

4 e: 3. In the process of treating powdered metal parts at oxide selected from the group consisting of stabilized elevated temperatures in the presence of metal furnace zirconia, alumina and titania.

parts, which powdered metal parts contain zinc volatile at the elevated temperature, the improvement which com- References Cited in file of this Patent prises coating said metal parts with a refractory metal 5 UNITED STATES PATENTS 1,852,162 Harris et a1. Apr. 5, 1932 4. In the process of treating powdered metal parts at 2,174 597 Pyster t 1 Oct 3, 1939 elevated temperatures in the presence of metal furnace 2,220,701 Benner et 1 Nov, 5, 1940 parts, which powdered metal parts contain zinc volatile 2,775,531 Montgomery et al Dec. 25, 1956 at the elevated temperature, the improvement which cOrn- 10 2,7 81,636 Brandes et a1. Feb. 19, 1957 prises coating Said metal parts With a refractory metal 2,824,794 Hathaway Feb. 25, 1958 

2. IN THE PROCESS OF TREATING MATERIALS AT ELEVATED TEMPERATURES IN THE PRESENCE OF METAL PARTS, WHICH MATERIALS CONTAIN METALS VOLATILE AT SAID ELEVATED TEMPERATURE AND CORROSIVE TO THE METAL PARTS, THE IMPROVEMENT WHICH COMPRISES COATING SAID METAL PARTS WITH REFRACTORY METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF STABILIZED ZIRCONIA, ALUMINUM AND TITANIA. 